Calibrating correlation between PES and off-track displacement by processing high frequency signal in spiral track

ABSTRACT

A method is disclosed for writing product servo sectors to the disk of a disk drive by demodulating spiral tracks recorded on the disk. Each spiral track comprises a high frequency signal interrupted at a predetermined interval by a sync mark. The high frequency signal is demodulated into a plurality of servo burst signals, and a position error signal is generated from the servo burst signals. A correlation between the position error signal and an off-track displacement of a head is calibrated, for example, by moving the head radially over the disk until the servo burst signals attain a first predetermined relationship, and then calibrating the correlation in response to the corresponding position error signal.

This application is a continuation-in-part of co-pending patentapplication Ser. No. 10/769,387 filed on Jan. 31, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to disk drives for computer systems. Moreparticularly, the present invention relates to calibrating correlationbetween PES and off-track displacement by processing high frequencysignal in spiral track.

2. Description of the Prior Art

When manufacturing a disk drive, product servo sectors 2 ₀-2 _(N) arewritten to a disk 4 which define a plurality of radially-spaced,concentric data tracks 6 as shown in the prior art disk format ofFIG. 1. Each product servo sector (e.g., servo sector 2 ₄) comprises apreamble 8 for synchronizing gain control and timing recovery, a syncmark 10 for synchronizing to a data field 12 comprising coarse headpositioning information such as a track number, and servo bursts 14which provide fine head positioning information. During normal operationthe servo bursts 14 are processed by the disk drive in order to maintaina head over a centerline of a target track while writing or readingdata. In the past, external servo writers have been used to write theproduct servo sectors 2 ₀-2 _(N) to the disk surface duringmanufacturing. External servo writers employ extremely accurate headpositioning mechanics, such as a laser interferometer, to ensure theproduct servo sectors 2 ₀-2 _(N) are written at the proper radiallocation from the outer diameter of the disk to the inner diameter ofthe disk. However, external servo writers are expensive and require aclean room environment so that a head positioning pin can be insertedinto the head disk assembly (HDA) without contaminating the disk. Thus,external servo writers have become an expensive bottleneck in the diskdrive manufacturing process.

The prior art has suggested various “self-servo” writing methods whereinthe internal electronics of the disk drive are used to write the productservo sectors independent of an external servo writer. For example, U.S.Pat. No. 5,668,679 teaches a disk drive which performs a self-servowriting operation by writing a plurality of spiral tracks to the diskwhich are then processed to write the product servo sectors along acircular path. The spiral tracks are written “open loop” by seeking thehead from an outer diameter of the disk to an inner diameter of thedisk. The disk drive calibrates acceleration/deceleration impulses toseek the head from the outer to inner diameter in a desired amount oftime. Accurate radial positioning of the spiral tracks assumes thecalibration process is accurate and that the calibratedacceleration/deceleration impulses will generate a repeatable responseover multiple seeks. However, the calibration process will inevitablyexhibit some degree of error and the dynamics of the disk drive willchange between seeks inducing errors in the radial position of thespiral tracks. Dynamic errors which degrade the spiral tracks writtenduring an open loop seek include vibration of the HDA, flutter andnon-repeatable run-out of the disk and spindle bearings, stiction andnon-repeatable run-out of the pivot bearings, windage on the head andarm, and flex circuit bias, windage and vibration. Errors in writing thespiral tracks will propagate to the product servo sectors, therebydegrading the operating performance of the disk drive and reducing themanufacturing yield.

In the '679 patent, each spiral track is written to the disk as a highfrequency signal (with missing bits), wherein the position error signal(PES) for tracking is generated relative to time shifts in the detectedlocation of the spiral tracks. In addition, the '679 patent generates aservo write clock by synchronizing a phase-locked loop (PLL) to themissing bits in the spiral tracks. In order to initially synchronize thePLL to the missing bits the head must servo accurately in a circularpath since PLL phase error can occur due to actual timing errors orradial tracking errors. Conversely, PLL phase errors cause radialtracking errors making it difficult to simultaneously maintain the headin a circular path by servoing on the spiral tracks while attempting tosynchronize the PLL to the missing bits.

There is, therefore, a need to improve the servo writing process for adisk drive by reducing the bottleneck and expense of external servowriters while maintaining adequate operating performance andmanufacturing yield.

SUMMARY OF THE INVENTION

The present invention may be regarded as a method associated with a diskdrive. The disk drive comprises control circuitry and a head diskassembly (HDA) comprising a disk, an actuator arm, a head connected to adistal end of the actuator arm. A voice coil motor rotates the actuatorarm about a pivot to position the head radially over the disk. The diskcomprises a plurality of spiral tracks, wherein each spiral trackcomprises a high frequency signal interrupted at a predeterminedinterval by a sync mark. The head internal to the disk drive is used toread the spiral tracks to generate a read signal. Burst demodulationcircuitry demodulates the read signal representing the high frequencysignal between the sync marks in one of the spiral tracks into aplurality of respective servo burst signals. The servo burst signalsgenerated from reading the spiral track are processed to generate aposition error signal (PES) used to maintain the head along asubstantially circular target path. A correlation between the positionerror signal and an off-track displacement of the head is calibrated.

In one embodiment, the head internal to the disk drive is used to writeproduct servo sectors along the circular target path, wherein eachproduct servo sector comprises a plurality of servo bursts.

In another embodiment, the correlation between the position error signaland an off-track displacement of the head is calibrated by moving thehead radially over the disk until the servo burst signals generated fromreading the spiral tracks attain a first predetermined relationship,generating a first position error signal from the servo burst signals,and calibrating the correlation in response to the first position errorsignal.

In yet another embodiment, the burst demodulation circuitry comprises anintegrator.

In still another embodiment, the correlation between the position errorsignal and the off-track displacement of the head is calibrated byfurther moving the head radially over the disk until the servo burstsignals generated from reading the spiral tracks attain a secondpredetermined relationship, generating a second position error signalfrom the servo burst signals, calibrating the correlation in response tothe first and second position error signals. In yet another embodiment,a first normalized scaling factor is computed as a ratio between a firstexpected head displacement and the first position error signal, a secondnormalized scaling factor is computed as a ratio between a secondexpected head displacement and the second position error signal, and thecorrelation is calibrated in response to the first and second normalizedscaling factors. In one embodiment, the first normalized scaling factoris weighted using a first weight to generate a first weighted normalizedscaling factor, the second normalized scaling factor is weighted using asecond weight to generate a second weighted normalized scaling factor,wherein the second weight is different than the first weight, and thecorrelation is calibrated in response to the first and second weightednormalized scaling factors.

In one embodiment, the control circuitry within the disk drive comprisesthe burst demodulation circuitry, and in another embodiment, an externalproduct servo writer comprises the burst demodulation circuitry.

The present invention may also be regarded as a disk drive comprising adisk having a plurality of spiral tracks, wherein each spiral trackcomprises a high frequency signal interrupted at a predeterminedinterval by a sync mark. A head is connected to a distal end of anactuator arm, and a voice coil motor rotates the actuator arm about apivot to position the head radially over the disk. The disk drivefurther comprises burst demodulation circuitry. The head internal to thedisk drive is used to read the spiral tracks to generate a read signal.The burst demodulation circuitry demodulates the read signalrepresenting the high frequency signal between the sync marks in one ofthe spiral tracks into a plurality of respective servo burst signals.The servo burst signals generated from reading the spiral track areprocessed to generate a position error signal (PES) used to maintain thehead along a substantially circular target path. A correlation betweenthe position error signal and an off-track displacement of the head iscalibrated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art disk format comprising a plurality of radiallyspaced, concentric tracks defined by a plurality of product servosectors.

FIGS. 2A and 2B illustrate an embodiment of the present inventionwherein an external spiral servo writer is used to write a plurality ofreference servo sectors and a plurality of spiral tracks to the disk foruse in writing product servo sectors to the disk.

FIG. 3A shows an embodiment of the present invention wherein a servowrite clock is synchronized by clocking a modulo-N counter so that itreaches terminal count at the frequency of sync marks in the referenceservo sectors.

FIG. 3B shows an eye pattern generated by reading the spiral track,including the sync marks in the spiral track.

FIG. 4 illustrates an embodiment of the present invention whereinsynchronization of the servo write clock is maintained from a coarsetiming recovery measurement generated in response to the sync marksrecorded in the spiral tracks and a fine timing recovery measurementgenerated in response to the high frequency signal in the spiral tracks.

FIGS. 5A-5B illustrate how in one embodiment the control circuitry fordemodulating the servo bursts in product servo sectors is also used todemodulate the high frequency signal in the spiral tracks as servobursts to generate the PES for tracking.

FIGS. 6A-6B show an embodiment of the present invention for calibratingthe correlation between the PES generated from reading the spiral tracksand off-track displacement.

FIG. 7 shows an embodiment of the present invention wherein an externalproduct servo writer is used to process the spiral tracks in order towrite the product servo sectors to the disk.

FIG. 8 shows an embodiment of the present invention wherein an externalspiral servo writer is used to write the reference servo sectors and thespiral tracks, and a plurality of external product servo writers writethe product servo sectors for the HDAs output by the external spiralservo writer.

FIG. 9 shows an embodiment of the present invention wherein an externalspiral servo writer is used to write the reference servo sectors and thespiral tracks, and the control circuitry within each product disk driveis used to write the product servo sectors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 2A and 2B illustrate an embodiment of the present invention forwriting product servo sectors to a disk 16 of a disk drive 18. The diskdrive 18 comprises control circuitry 20 and a head disk assembly (HDA)22 comprising the disk 16, an actuator arm 24, a head 26 connected to adistal end of the actuator arm 24, and a voice coil motor 28 forrotating the actuator arm 24 about a pivot to position the head 26radially over the disk 16. A head positioning pin 30 of an externalspiral servo writer 32 is inserted into the HDA 22, wherein the headpositioning pin 30 for engaging the actuator arm 24. The external spiralservo writer 32 derives a radial location of the head 26 and actuatesthe head positioning pin 30 in response to the radial location of thehead 26 in a closed loop system to rotate the actuator arm about thepivot in order to position the head radially over the disk 16 whilewriting a plurality of spiral tracks 36 ₀-36 _(N), wherein each spiraltrack comprises a high frequency signal interrupted at a predeterminedinterval by a sync mark.

The head positioning pin 30 is then removed from the HDA 22, and thehead 26 internal to the disk drive 18 is used to read the high frequencysignal 38 in the spiral tracks 36 ₀-36 _(N). A position error signal isgenerated to maintain the head 26 along a substantially circular targetpath while reading the sync marks 40 in the spiral tracks 36 ₀-36 _(N)to generate a spiral sync mark detect signal, wherein a servo writeclock is synchronized in response to the spiral sync mark detect signal.The servo write clock and the head 26 internal to the disk drive 18 arethen used to write the product servo sectors along the circular targetpath.

In one embodiment, the external spiral servo writer 32 also writes aplurality of reference servo sectors 34 (FIG. 2B) in a substantiallycircular reference path, each reference servo sector 34 comprising async mark 10 and a plurality of servo bursts 14 (FIG. 1). The servobursts 14 in the reference servo sectors 34 are then read using the head26 internal to the disk drive 18 to generate a position error signalused to maintain the head 26 along the circular reference path whilereading the sync marks 10 in the reference servo sectors 34 to generatea reference sync mark detect signal which is used to initiallysynchronize the servo write clock.

In the embodiment of FIG. 2A, the external spiral servo writer 32comprises a head positioner 42 for actuating the head positioning pin 30using sensitive positioning circuitry, such as a laser interferometer.Pattern circuitry 44 generates the data sequence written to the disk 16for the reference servo sectors 34 and the spiral tracks 36 ₀-36 _(N).The external spiral servo writer 32 writes a clock track 46 (FIG. 2B) atan outer diameter of the disk 16, and a clock head 48 is inserted intothe HDA 22 for reading the clock track 46 to generate a clock signal 50.Timing circuitry 52 in the external spiral servo writer 32 processes theclock signal 50 to enable the pattern circuitry 44 at the appropriatetime so that the reference servo sectors 34 and spiral tracks 36 ₀-36_(N) are written at the appropriate circumferential location. The clocksignal 50 also enables the pattern circuitry 44 to write the sync marks40 (FIG. 3B) within the spiral tracks 36 ₀-36 _(N) at the samecircumferential location from the outer diameter to the inner diameterof the disk 16. As described below with reference to FIG. 4, theconstant interval between sync marks 40 (independent of the radiallocation of the head 26) enables the servo write clock to maintainsynchronization.

In the embodiment of FIG. 2A, the entire disk drive 18 is shown as beinginserted into the external spiral servo writer 32. In an alternativeembodiment, only the HDA 22 is inserted into the external spiral servowriter 32.

After the external spiral servo writer 32 writes the reference servosectors 34 and the spiral tracks 36 ₀-36 _(N) to the disk 16, the headpositioning pin 30 and clock head 48 are removed from the HDA 22 and theproduct servo sectors are written to the disk 16. In one embodiment, thecontrol circuitry 20 within the disk drive 18 is used to process thereference servo sectors 34 and spiral tracks 36 ₀-36 _(N) in order towrite the product servo sectors to the disk 16. In an alternativeembodiment described below with reference to FIGS. 7 and 8, an externalproduct servo writer is used to process the reference servo sectors 34and spiral tracks 36 ₀-36 _(N) in order to write the product servosectors to the disk 16 during a “fill operation”.

At the beginning of the fill operation, the reference servo sectors 34are processed in order to synchronize the servo write clock. Thereference servo sectors 34 are processed similar to conventional productservo sectors. The circumferential location of the reference servosectors 34 is first determined by searching for the sync mark 10asynchronously. Once a sync mark 10 is detected, the reference servosectors 34 are detected synchronously by synchronizing a read clock tothe preamble 8 (FIG. 1) preceding the sync mark 10. The servo bursts 14in the reference servo sectors 34 are also demodulated in a conventionalmanner to generate a position error signal (PES) for use in maintaining(tracking) the head 26 along a circumferential path while reading thereference servo sectors 34. In one embodiment, several tracks ofreference servo sectors 34 are written to the disk 16 to facilitatefinding and tracking the reference servo sectors 34.

FIG. 3A shows an embodiment of the present invention wherein two spiraltracks are written between each reference servo sector (e.g., spiraltracks 36 ₀ and 36 ₁ written between reference servo sectors 34 ₀ and 34₁). Also shown in FIG. 3A is a saw-tooth waveform 54 representing thevalue of a modulo-N counter. The modulo-N counter is clocked by theservo write clock, and the frequency of the servo write clock isadjusted until the modulo-N counter reaches terminal count synchronouswith detecting the sync mark 10 in the reference servo sector 34. Theservo write clock may be generated using any suitable circuitry. In oneembodiment, the servo write clock is generated using a phase locked loop(PLL). As each sync mark 10 in the reference servo sectors 34 isdetected, the value of the modulo-N counter represents the phase errorfor adjusting the PLL. Once the modulo-N counter reaches terminal countsubstantially synchronous with detecting the sync marks 10 in thereference servo sectors 34, the servo write clock is coarsely locked tothe desired frequency for writing the product servo sectors to the disk.

After synchronizing the servo write clock in response to the referenceservo sectors, the spiral tracks 36 ₀-36 _(N) are read to generate thePES signal for tracking as well as to maintain synchronization of theservo write clock. FIG. 3B illustrates an “eye” pattern in the readsignal that is generated when the head 26 passes over a spiral track 36.The read signal representing the spiral track comprises high frequencytransitions 38 interrupted by sync marks 40. When the head 26 moves inthe radial direction, the eye pattern will shift (left or right) whilethe sync marks 40 remain fixed. The shift in the eye pattern (detectedfrom the high frequency signal 38) relative to the sync marks 40provides the off-track information for servoing the head 26.

The sync marks 40 in the spiral tracks 36 ₀-36 _(N) may comprise anysuitable pattern, and in one embodiment, a pattern that is substantiallyshorter than the sync mark 10 in the reference servo sectors 34.Referring again to FIG. 3A, when the sync marks 40 in the spiral tracks36 ₀-36 _(N) are detected, the value of the modulo-N counter is comparedto an expected value, and the resulting error represents the phase errorfor adjusting the PLL that generates the servo write clock. In oneembodiment, the PLL is updated when any one of the sync marks 40 withinthe eye pattern is detected. In this manner the multiple sync marks 40in each eye pattern (each spiral track crossing) provides redundancy sothat the PLL is still updated if one or more of the sync marks 40 aremissed due to noise in the read signal.

In one embodiment, the servo write clock is further synchronized bygenerating a timing recovery measurement from the high frequency signal38 between the sync marks 40 in the spiral tracks 36 ₀-36 _(N).Synchronizing the servo write clock to the high frequency signal 38helps maintain proper radial alignment (phase coherency) of the Graycoded track addresses in the product servo sectors. The timing recoverymeasurement may be generated in any suitable manner. In one embodiment,the servo write clock is used to sample the high frequency signal 38 andthe signal sample values are processed to generate the timing recoverymeasurement. The timing recovery measurement adjusts the phase of theservo write clock (PLL) so that the high frequency signal 38 is sampledsynchronously. In this manner, the sync marks 40 provide a coarse timingrecovery measurement and the high frequency signal 38 provides a finetiming recovery measurement for maintaining synchronization of the servowrite clock.

FIG. 4 illustrates how the product servo sectors 56 ₀-56 _(N) arewritten to the disk 16 after synchronizing the servo write clock usingthe reference servo sectors 34 ₀-34 _(N). In the embodiment of FIG. 4,the dashed lines represent the centerlines of the data tracks. The syncmarks in the spiral tracks 36 ₀-36 _(N) are written so that there is ashift of two sync marks in the eye pattern (FIG. 3B) between datatracks. In an alternative embodiment, the sync marks in the spiraltracks 36 ₀-36 _(N) are written so that there is a shift of N sync marksin the eye pattern between data tracks. In practice the width of thespiral tracks 36 ₀-36 _(N) in the embodiment of FIG. 4 will be proximatethe width of a data track. The spiral tracks 36 ₀-36 _(N) are shown inFIG. 4 as being wider than the width of a data track for illustrationpurposes.

The PES for maintaining the head 26 along a servo track (tracking) maybe generated from the spiral tracks 36 ₀-36 _(N) in any suitable manner.In one embodiment, the PES is generated by detecting the eye pattern inFIG. 3B using an envelope detector and detecting a shift in the enveloperelative to the sync marks 40. In one embodiment, the envelope isdetected by integrating the high frequency signal 38 and detecting ashift in the resulting ramp signal. In an alternative embodimentdisclosed below with reference to FIG. 5B, the high frequency signal 38between the sync marks 40 in the spiral tracks are demodulated as servobursts and the PES generated by comparing the servo bursts in a similarmanner as with the servo bursts 14 in the reference servo sectors 34₀-34 _(N).

Once the head 26 is tracking on a servo track, the product servo sectors56 ₀-56 _(N) are written to the disk using the servo write clock. Writecircuitry is enabled when the modulo-N counter reaches a predeterminedvalue, wherein the servo write clock clocks the write circuitry to writethe product servo sector 56 to the disk. The spiral tracks 36 ₀-36 _(N)on the disk are processed in an interleaved manner to account for theproduct servo sectors 56 ₀-56 _(N) overwriting a spiral track. Forexample, when writing the product servo sectors 56 ₁ to the disk, spiraltrack 36 ₂ is processed initially to generate the PES tracking error andthe timing recovery measurement. When the product servo sectors 56 ₁begin to overwrite spiral track 36 ₂, spiral track 36 ₃ is processed togenerate the PES tracking error and the timing recovery measurement.

FIGS. 5A-5B illustrate an embodiment of the present invention whereincontrol circuitry for demodulating the servo bursts in prior art productservo sectors is also used to demodulate the high frequency signal 38 inthe spiral tracks as servo bursts to generate the PES for tracking. FIG.5A shows the eye pattern of FIG. 3B, which is processed similar to theprior art product servo sector shown in FIG. 1. The first segment 38A ofthe high frequency signal in the eye pattern of FIG. 5A is processed asa preamble similar to the preamble 8 in FIG. 1 for synchronizing a readclock 58 generated by a read voltage controlled oscillator (VCO) 60. Thefirst sync mark 40A in the eye pattern is processed similar to the syncmark 10 in FIG. 1. The following segments 38B-38E of the high frequencysignal in the eye pattern are demodulated as servo bursts used togenerate the PES for tracking.

FIG. 5B shows example control circuitry for demodulating the prior artproduct servo sector of FIG. 1 as well as the eye pattern (FIG. 5A) ofthe spiral tracks 36. The embodiment employs a read VCO 60 and a writeVCO 62. The read VCO 60 generates a read clock 58 for sampling the readsignal 64 during normal operation when demodulating the product servosectors 54 and user data recorded on the disk. The write VCO 62generates the servo write clock 66 used to write the product servosectors 54 to the disk during the fill operation. The write VCO 62 isalso used to sample the read signal 64 when demodulating the servobursts from the high frequency signal 38 in the spiral tracks 36.

In one embodiment, the read clock 58 is also used to sample the readsignal 64 when reading the first segment 38A of the high frequencysignal representing the preamble as well as the first sync mark 40A inthe eye pattern (FIG. 5A) of the spiral tracks 36. The read clock 58 isselected by multiplexer 68 as the sampling clock 70 for sampling 72 theread signal 64. The read signal sample values 74 are processed by afirst timing recovery circuit 76, which generates a timing recoverysignal used to adjust the read VCO 60 until the read clock 58 issampling the preamble 38A synchronously. Once locked onto the preamble38A, a sync detector 78 is enabled for detecting the sync mark 40A inthe eye pattern. When the sync detector 78 detects the sync mark 40A, itactivates a sync detect signal 80. The first timing recovery circuit 76responds to the sync detect signal 80 by configuring the multiplexer 68over line 82 to select the servo write clock 66 as the sampling clock70. The first timing recovery circuit 76 enables a timer for timing aninterval between the sync mark 40A and the beginning of the A servoburst 38B in the eye pattern. When the timer expires, the first timingrecovery circuit 76 enables a burst demodulator 84 over line 86 fordemodulating the A, B, C and D servo bursts in the eye pattern from theread signal sample values 74.

In one embodiment, the burst demodulator 84 rectifies and integrates therectified read signal sample values 74 representing the respective A, B,C and D servo bursts to generate respective servo burst signals 88 whichcorrespond to integrating the A, B, C and D servo bursts 14 in the priorart product servo sector of FIG. 1. A PES generator 90 processes theservo burst signals 88 to generate a PES signal 92 used for tracking.The PES generator 90 may compare the servo burst signals 88 to generatethe PES signal 92 using any suitable algorithm when demodulating theservo bursts in either the prior art product servo sectors of FIG. 1 orthe eye pattern of FIG. 5A. In one embodiment, the PES signal 92 whenreading the eye pattern of FIG. 5A is generated according to(A−D)/(A+D). In this embodiment, evaluating the servo bursts near theedges of the eye pattern increases the sensitivity of the PESmeasurement. This is because deviations in the radial location of thehead 26 cause a more precipitous change in the servo burst values at theedges of the eye pattern as compared to the servo burst values near thecenter of the eye pattern.

In the embodiment of FIG. 5B, a control signal C/S 94 configures thefirst timing recovery circuit 76, the sync detector 78, and the PESgenerator 90 depending on whether the control circuitry is configuredfor demodulating the product servo sector (prior art product servosector of FIG. 1) or the spiral tracks. The first timing recoverycircuit 76 adjusts the timing between the detection of the sync mark (10in FIGS. 1 and 40A in FIG. 5A) and the beginning of the A servo burst(14 in FIGS. 1 and 38B in FIG. 5A). The sync detector 78 adjusts thetarget sync pattern depending on whether the sync mark 10 in the productservo sector is being detected or the sync mark 40A in the eye patternof the spiral track. The PES generator 90 adjusts the algorithm forcomparing the servo burst signals 88 depending on whether the servobursts 14 in the product servo sectors are being demodulated or theservo bursts 38B-38E in the eye pattern of the spiral track are beingdemodulated.

The control circuitry in the embodiment of FIG. 5B further comprises asecond timing recovery circuit 96 for generating a timing recoverymeasurement that controls the write VCO 62 for generating the servowrite clock 66. The second timing recovery circuit 96 comprises themodulo-N counter which is synchronized to the sync marks in thereference servo sectors 34 as shown in FIG. 3A. When servoing on thespiral tracks 36, the second timing recovery circuit 96 enables a syncmark detection window over line 98 commensurate with the modulo-Ncounter approaching a value corresponding to the expected occurrence ofa sync mark 40 in a spiral track. When a sync mark 40 is actuallydetected over line 80, the second timing recovery circuit 96 generates acoarse timing recovery measurement as the difference between theexpected value of the module-N counter and the actual value. Whenreading the high frequency signal 38 in the spiral tracks, the secondtiming recovery circuit 96 generates a fine timing recovery measurementusing any suitable timing recovery algorithm. For example, the finetiming recovery measurement can be generated using a suitable timinggradient, a suitable trigonometric identity, or a suitable digitalsignal processing algorithm such as the Discrete Fourier Transform(DFT). The coarse and fine timing recovery measurements are combined andused to adjust the write VCO 62 in order to maintain synchronization ofthe servo write clock 66.

The servo write clock 66 is applied to write circuitry 100 used to writethe product servo sectors 56 to the disk during the fill operation. Thesecond timing recovery circuit 96 generates a control signal 102 forenabling the write circuitry 100 at the appropriate time so that theproduct servo sectors 56 are written at the appropriate circumferentiallocation from the outer diameter of the disk to the inner diameter ofthe disk. In one embodiment, the control signal 102 enables the writecircuitry 100 each time the module-N counter reaches a predeterminedvalue so that the product servo sectors 56 form servo wedges asillustrated in FIG. 1 and FIG. 4.

Although the first timing recovery circuit 76 shown in FIG. 5B adjuststhe frequency of the read clock 58, any suitable timing recoverytechnique may be employed. In an alternative embodiment, interpolatedtiming recovery is employed. With interpolated timing recovery the isread signal 64 is sampled asynchronously and interpolated to generatethe synchronous sample values 74. In addition, the reference servosectors 34 may comprise any suitable sync mark recorded at any suitablelocation within the reference servo sector. In one embodiment, thereference servo sector comprises multiple sync marks to provideredundancy similar to the eye pattern of FIG. 5A.

FIGS. 6A and 6B illustrate an embodiment of the present invention forcalibrating the correlation between the PES generated from demodulatingthe spiral tracks 36 and the off-track displacement of the head 26. Thesegments 38B-38E of the high frequency signal in the spiral tracks 36are demodulated as servo bursts to generate corresponding servo burstsignals A, B, C and D. A PES is generated by comparing the servo burstsignals according to any suitable algorithm, such as (A−D)/(A+D). Asshown in FIG. 6A, when the head 26 is on track a predeterminedrelationship between the servo burst signals (e.g., A=D) generates apredetermined value for the PES (e.g., zero). The head 26 is then movedaway from the center of the track until the servo burst signals reach asecond predetermined relationship (e.g., B=D) as shown in FIG. 6B. Whenthe servo burst signals reach the second predetermined relationship, theshift in the eye pattern relative to the sync marks 40A-40D is known andtherefore the amount of off-track displacement is known. Measuring thePES when the servo burst signals reach the second predeterminedrelationship provides the correlation (assuming a linear relationship)between the PES and the amount of off-track displacement.

In one embodiment, a plurality of normalized scaling factors arecomputed as a ratio between the expected head displacement value and thePES for a number of different relationships between the servo burstsignals. That is, the PES is measured at a plurality of differentoff-track displacements for the head wherein the servo burst signalsreach a number of corresponding relationships (e.g., A=C, A=B, B=D adC=D). A normalized scaling factor Norm_(i) is then computed for the Ncorresponding relationships:

${{Norm}_{i} = \frac{{ExpHeadDisp}_{i}}{{PES}_{i}}},{{{for}\mspace{14mu} i} = 1},\ldots\mspace{14mu},N$In one embodiment, the normalized scaling factors are weighted bycorresponding weights w₁-w_(N) to generate a nominal scaling factorNomNorm:NomNorm=w ₁·Norm₁ + . . . +w _(N)·Norm_(N)The nominal scaling factor NomNorm is then used to scale the PES whilewriting the product servo sectors to the disk, thereby achieving thedesired off-track displacement measurement for servoing the head. In oneembodiment, the weights w₁-w_(N) are selected to de-emphasize thecontribution of the corresponding normalized scaling factors thatprovide a less reliable correlation between off-track displacement andPES. That is, certain of the relationships of the servo burst signals(e.g., A=B and C=D) may provide a less reliable correlation betweenoff-track displacement and PES, and therefore the corresponding weightis reduced to de-emphasize the corresponding normalized scaling factorin the computation of the nominal scaling factor NomNorm. In anotherembodiment, each normalized scaling factor Norm_(i) is generated over anumber of revolutions of the disk in order to average out noise from thePES measurement.

FIG. 7 shows an embodiment of the present invention wherein afterwriting the reference servo sectors 34 and spiral tracks 36 ₀-36 _(N) tothe disk 16 (FIGS. 2A-2B), the HDA 22 is inserted into an externalproduct servo writer 104 comprising suitable circuitry for reading andprocessing the reference servo sectors 34 and the spiral tracks 36 ₀-36_(N) in order to write the product servo sectors 56 ₀-56 _(N) to thedisk 16. The external product servo writer 104 comprises a read/writechannel 106 for interfacing with a preamp 108 in the HDA 22. The preamp108 amplifies a read signal emanating from the head 26 over line 110 togenerate an amplified read signal applied to the read/write channel 106over line 112. The read/write channel 106 comprises the circuitry ofFIG. 5B for generating the servo burst signals 88 applied to a servocontroller 114. The servo controller 114 processes the servo burstsignals 88 to generate the PES 92. The PES 92 is processed to generate aVCM control signal applied to the VCM 28 over line 116 in order tomaintain the head 26 along a circular path while writing the productservo sectors 56 ₀-56 _(N). The servo controller 114 also generates aspindle motor control signal applied to a spindle motor 118 over line120 to maintain the disk 16 at a desired angular velocity. Controlcircuitry 122 processes information received from the read/write channel106 over line 124 associated with the spiral tracks (e.g., timinginformation) and provides the product servo sector data to theread/write channel 106 at the appropriate time. The product servo sectordata is provided to the preamp 108, which modulates a current in thehead 26 in order to write the product servo sectors 56 ₀-56 _(N) to thedisk 16. The control circuitry 122 also transmits control informationover line 126 to the servo controller 114 such as the target servo trackto be written. After writing the product servo sectors 56 ₀-56 _(N) tothe disk 16, the HDA 22 is removed from the external product servowriter 104 and a printed circuit board assembly (PCBA) comprising thecontrol circuitry 20 (FIG. 2A) is mounted to the HDA 22.

In one embodiment, the external product servo writer 104 of FIG. 7interfaces with the HDA 22 over the same connections as the controlcircuitry 20 to minimize the modifications needed to facilitate theexternal product servo writer 104. The external product servo writer 104is less expensive than a conventional servo writer because it does notrequire a clean room or sophisticated head positioning mechanics. In anembodiment shown in FIG. 8, a plurality of external product servowriters 104 ₀-104 _(N) process the HDAs 22 _(i)-22 _(i+N) output by anexternal spiral servo writer 32 in order to write the product servosectors less expensively and more efficiently than a conventional servowriter. In an alternative embodiment shown in FIG. 9, an external spiralservo writer 32 is used to write the reference servo sectors and thespiral tracks, and the control circuitry 20 within each product diskdrive 18 _(i)-18 _(i+N) is used to write the product servo sectors.

1. A method associated with a disk drive, the disk drive comprisingcontrol circuitry and a head disk assembly (HDA) comprising a disk, anactuator arm, a head connected to a distal end of the actuator arm, anda voice coil motor for rotating the actuator arm about a pivot toposition the head radially over the disk, the disk comprising aplurality of spiral tracks, each spiral track comprising a highfrequency signal interrupted at a predetermined interval by a sync mark,the method comprising the steps of: (a) using the head internal to thedisk drive to read the spiral tracks to generate a read signal; (b)using burst demodulation circuitry to demodulate the read signalrepresenting the high frequency signal between the sync marks in one ofthe spiral tracks into a plurality of respective servo burst signals;(c) processing the servo burst signals generated from reading the spiraltrack to generate a position error signal (PES) used to maintain thehead along a substantially circular target path; and (d) calibrating acorrelation between the position error signal and an off-trackdisplacement of the head.
 2. The method as recited in claim 1, furthercomprising the step of using the head internal to the disk drive towrite product servo sectors along the circular target path, wherein eachproduct servo sector comprises a plurality of servo bursts.
 3. Themethod as recited in claim 1, wherein the step of calibrating thecorrelation between the position error signal and an off-trackdisplacement of the head comprises the steps of: moving the headradially over the disk until the servo burst signals generated fromreading the spiral tracks attain a first predetermined relationship;generating a first position error signal from the servo burst signals;and calibrating the correlation in response to the first position errorsignal.
 4. The method as recited in claim 1, wherein the burstdemodulation circuitry comprises an integrator.
 5. The method as recitedin claim 1, wherein the step of calibrating the correlation between theposition error signal and the off-track displacement of the head furthercomprises the steps of: moving the head radially over the disk until theservo burst signals generated from reading the spiral tracks attain asecond predetermined relationship; generating a second position errorsignal from the servo burst signals; and calibrating the correlation inresponse to the first and second position error signals.
 6. The methodas recited in claim 5, wherein the step of calibrating the correlationbetween the position error signal and the off-track displacement of thehead further comprises the steps of: computing a first normalizedscaling factor as a ratio between a first expected head displacement andthe first position error signal; computing a second normalized scalingfactor as a ratio between a second expected head displacement and thesecond position error signal; and calibrating the correlation inresponse to the first and second normalized scaling factors.
 7. Themethod as recited in claim 6, wherein the step of calibrating thecorrelation between the position error signal and the off-trackdisplacement of the head further comprises the steps of: weighting thefirst normalized scaling factor using a first weight to generate a firstweighted normalized scaling factor; weighting the second normalizedscaling factor using a second weight to generate a second weightednormalized scaling factor, wherein the second weight is different thanthe first weight; and calibrating the correlation in response to thefirst and second weighted normalized scaling factors.
 8. The method asrecited in claim 1, wherein the control circuitry within the disk drivecomprises the burst demodulation circuitry.
 9. The method as recited inclaim 1, wherein an external product servo writer comprises the burstdemodulation circuitry.
 10. A disk drive comprising: (a) a diskcomprising a plurality of spiral tracks, wherein each spiral trackcomprises a high frequency signal interrupted at a predeterminedinterval by a sync mark; (b) an actuator arm; (c) a head connected to adistal end of the actuator arm; (d) a voice coil motor for rotating theactuator arm about a pivot to position the head radially over the disk;(e) burst demodulation circuitry; and (f) control circuitry for: usingthe head internal to the disk drive to read the spiral tracks togenerate a read signal; using the burst demodulation circuitry todemodulate the read signal representing the high frequency signalbetween the sync marks in one of the spiral tracks into a plurality ofrespective servo burst signals; processing the servo burst signalsgenerated from reading the spiral track to generate a position errorsignal (PES) used to maintain the head along a substantially circulartarget path; calibrating a correlation between the position error signaland an off-track displacement of the head.
 11. The disk drive as recitedin claim 10, wherein the control circuitry for using the head internalto the disk drive to write product servo sectors along the circulartarget path, wherein each product servo sector comprises a plurality ofservo bursts.
 12. The disk drive as recited in claim 10, wherein thecontrol circuitry for calibrating the correlation between the positionerror signal and an off-track displacement of the head by: moving thehead radially over the disk until the servo burst signals generated fromreading the spiral tracks attain a first predetermined relationship;generating a first position error signal from the servo burst signals;and calibrating the correlation in response to the first position errorsignal.
 13. The disk drive as recited in claim 10, wherein the burstdemodulation circuitry comprises an integrator.
 14. The disk drive asrecited in claim 10, wherein the control circuitry calibrates thecorrelation between the position error signal and the off-trackdisplacement of the head by further: moving the head radially over thedisk until the servo burst signals generated from reading the spiraltracks attain a second predetermined relationship; generating a secondposition error signal from the servo burst signals; and calibrating thecorrelation in response to the first and second position error signals.15. The disk drive as recited in claim 14, wherein the control circuitrycalibrates the correlation between the position error signal and theoff-track displacement of the head by further: computing a firstnormalized scaling factor as a ratio between a first expected headdisplacement and the first position error signal; computing a secondnormalized scaling factor as a ratio between a second expected headdisplacement and the second position error signal; and calibrating thecorrelation in response to the first and second normalized scalingfactors.
 16. The disk drive as recited in claim 15, wherein the controlcircuitry calibrates the correlation between the position error signaland the off-track displacement of the head by further: weighting thefirst normalized scaling factor using a first weight to generate a firstweighted normalized scaling factor; weighting the second normalizedscaling factor using a second weight to generate a second weightednormalized scaling factor, wherein the second weight is different thanthe first weight; and calibrating the correlation in response to thefirst and second weighted normalized scaling factors.